The present invention relates to a planar lighting device used in a liquid crystal display or the like.
A liquid crystal display uses a planar lighting device (a backlight unit) which illuminates a liquid crystal display panel by irradiation with light from the back side of the liquid crystal display panel. The backlight unit is configured using a light guide plate for diffusing light emitted from an illumination light source to illuminate the liquid crystal display panel and parts such as a prism sheet and a diffusion sheet for making outgoing light from the light guide plate uniform.
Currently, large-size liquid crystal televisions predominantly use a so-called underneath type backlight unit including a light guide plate disposed immediately above an illumination light source. This type of backlight unit ensures uniform light amount distribution and necessary luminance by disposing a plurality of cold cathode tubes used as light sources behind the liquid crystal display panel and providing the inside of the backlight unit with white reflection surfaces.
However, the underneath type backlight unit requires a thickness of about 30 mm in a direction perpendicular to the liquid crystal display panel in order to make the light amount distribution uniform, and accordingly, further reduction in thickness is difficult to achieve.
On the other hand, an exemplary backlight unit that allows the thickness reduction includes one using a light guide plate which guides light, which is emitted from an illumination light source and caused to enter from a surface, in predetermined directions, and emits the guided light through a light exit surface that is different from the surface through which the light is caused to enter.
As a backlight unit using such a light guide plate, a backlight unit of a type using a plate-like light guide member in which scattering particles for scattering light are dispersed in a transparent resin and which causes light to enter from the side surface thereof and causes light to exit through the front surface thereof has been proposed.
For example, JP 07-036037 A discloses a light-scattering and guiding light source device which includes a light-scattering light guide member having at least one light incidence surface region and at least one light exit surface region and light source means for causing light to enter from the light incidence surface region and in which the light-scattering light guide member has a region in which the thickness thereof tends to decrease with increasing distance from a light incidence surface.
JP 08-248233 A discloses a planar light source device including a light-scattering light guide member, a prism sheet disposed on the side closer to a light exit surface of the light-scattering light guide member, and a reflector disposed on the side closer to the rear surface of the light-scattering light guide member. JP 08-271739 A discloses a liquid crystal display including a light exit direction correcting element which is formed of a plate-like optical material and which has a light incidence surface having repeated undulations of a prism train shape and a light exit surface having a light diffusing property. JP 11-153963 discloses a light source device including a light-scattering light guide member having scattering ability therein and light supply means for supplying light from an end surface portion of the light-scattering light guide member.
In addition to the above-mentioned light guide plates, a light guide plate in which the thickness of an intermediate portion is larger than the thicknesses of an end portion of a light incidence side and an end portion of an opposed side, a light guide plate having a reflecting surface which is inclined in a direction in which the thickness increases with increasing distance from a light incidence portion, and a light guide plate having a shape in which the distance between a front surface portion and a rear surface portion is the smallest in a light incidence portion and the thickness at a position most distant from the light incidence portion is the largest have also been proposed as a light guide plate (for example, see JP 2003-090919 A, JP 2004-171948 A, JP 2005-108676 A, JP 2005-302322 A, and JP 08-220346 A).
Furthermore, JP 2009-117349 A discloses a lighting device in which a light exit surface of a light guide member is a concave surface. JP 2009-117357 A discloses a light guide plate in which a light exit surface of the light guide plate is a curved surface convex to downward (that is, the light exit surface is a concave surface).
JP 2009-117357 A discloses a light guide plate including two layers, in which an interface between the first layer and the second layer is an inclined surface inclined in the direction of approaching a light exit surface with increasing distance from an end portion toward the center of the light guide plate (that is, the cross-sectional shape of the light guide plate is an equilateral triangle).
Furthermore, JP 4127897 B (JP 11-345512 A) discloses a planar light source device including a plate-like member in which at least one non-scattering light guide region and at least one scattering light guide region containing particles having different refractive indices uniformly dispersed in the same material as the non-scattering light guide region overlap with each other. In the planar light source device, a distribution of the amount of light emitted from a main surface is controlled by mounting a light source lamp on an end face of the plate-like member and locally adjusting the particle concentration by using the thicknesses of both the regions; and the scattering light guide region is a light guide block having a convex shape while the non-scattering light guide region is a light guide block having a concave shape corresponding to the light guide block having the convex shape.